The present invention relates to a method for igniting fuel in a combustion chamber of a thermal turbomachine.
In order to set a continuous chemical reaction in motion, the combustion chambers of furnaces and gas turbine plants are ignited generally by what are known as ignition aids. In this respect, in particular, electric spark or glow plugs are known, but a gas flame (ignition torch) is also often used for the ignition.
In this context, auxiliary burners are usually ignited first, these finally igniting the actual burners. Where gas turbines are concerned, the main fuel is sometimes also used for the ignition burners, that is to say fuel oil, kerosene or natural gas, depending on the application. Occasionally, however, easily storable auxiliary gases, such as, for example, propane, are also employed.
The ignition success achieved by means of the ignition aids depends on a multiplicity of factors. These include, inter alia, the inlet conditions of the combustion air, such as pressure, temperature and velocity, the ignition fuel used and its conditions, the geometry and nature of the combustion chamber and, finally, also the composition of the combustion air.
The ignition of the combustion chambers in thermal turbomachines, that is to say, for example, in gas turbines, by means of a spark plug or glow plug is described, inter alia, by A. Lefebvre in xe2x80x9cGas Turbine Combustionxe2x80x9d (Arthur H. Lefebvre: Gas Turbine Combustion pages 222-225 and pages 232/233, Hemisphere Publishing Corporation, 1983).
A prerequisite for the functioning of the ignition device is, of course, that conditions prevail at the ignitor which make it possible to generate a stable and sufficiently strong ignition flame. In this case, in particular, mention must be made of the setting of the fuel/air ratio at the ignitor and of an expedient flow velocity in the region of the ignitor.
Particularly in the combustion chambers of gas turbines, these requirements often can be fulfilled only by the use of special ignition burners. The placing of the ignitor directly in the combustion space is automatically ruled out on account of the abovementioned criteria. Often, therefore, the ignitor is accommodated in a small ignition space, so that only a small volume has to be inflamed by initial ignition. A stable ignition flame is generated successively in a flame tube located downstream of this ignition space.
Such an ignition system for gas turbines is known from the publication EP-A1-992 661. Other ignitors are known, for example, from the publications U.S. Pat. Nos. 2,967,244, 3,404,939, 5,344,310 and 5,368,474 or from the international publication WO96/39578. A burner with an ignition device for gaseous fuels and oxygen is also described in the publication DD-241 457. Furthermore, a method for igniting the main flame of a combustion chamber is known from the publication DE-A1-4 320 429.
In ignition burners of this type, it is customary to use for the ignition flame lean mixtures with a fuel/air mixture ratio "PHgr"overall=1/xcexoverall less than 1, xcexoverall representing the air ratio balanced over the entire control space, that is to say a lean mixture being ignited. Locally, of course, mixture ratios from the rich to the lean stability limit are produced.
The aim of the invention is to avoid the disadvantages mentioned. The object on which the invention is based is to provide a method for igniting a thermal turbomachine, in which case a diffusion ignition flame with very good ignition properties is to be achieved over a wide stability range.
According to the invention, in a method for igniting a thermal turbomachine according to the preamble of claim 1, the object is achieved in that, prior to the ignition of the combustion chamber, an overall fuel/air mixture ratio "PHgr"overall=1 /xcexoverall greater than 1 prevails in the flame tube of the ignition device, with xcexoverall being the overall air ratio, the fuel/air mixture is ignited by means of the ignitor in the ignition space, and the flame and the remaining fuel/air mixture runs from the ignition space through the flame tube to the combustion chamber and ignites the combustion chamber there. In a particular embodiment, an overall fuel/air mixture ratio "PHgr"overall of 1 to 34 is set.
By the inflow velocity of the two media, air/fuel, being suitably selected higher than the flame velocity, the ignited fuel/air mixture then runs through the flame tube in the direction of the combustion chamber. There, by virtue of a cross-sectional widening, the flame is stabilized. The ignition of the main burners is subsequently carried out by means of the ignition flame burning in the combustion chamber. A method of this type leads to a highly stable diffusion flame which additionally has the property that, by being overenriched when it comes into contact with the air of the combustion chamber, it has good ignition properties, in that not only the hot gases of the ignition lance, but the additionally inflamed zones of the encased excess fuel burn in a diffusion-like manner.
In the type of ignition burner used, it is advantageous that the air surrounds the fuel annularly prior to ignition and the fuel is thereby locally diluted from outside. It may also be envisaged that the air is injected into the flame tube via a plurality of bores. Stabilization of the ignition flame may be carried out, as is known, by means of a free jet, by a borda jump or by vortex structures.
For the better ignition of the rich mixture which is present in the flame tube, it is advantageous, for the initial ignition of the ignition flame in the ignition space, to generate a lean fuel/air mixture and thus locally dilute the rich mixture there.
It may be envisaged, furthermore, that additional oxygen is added prior to and during the ignition of the ignition air. This leads to a reduction in the minimum ignition energy for igniting the mixture. This is of particular technical significance in the ignition of fuels which contain the especially ignitable H2.
The flame tube can be used only once for igniting the combustion chamber and fuel and/or air can thereafter be supplied to the combustion chamber via the flame tube.